Add guidelines for managing issues to maintenance.md (#1301)
Add guidelines for managing issues to maintenance.md (#1301)

Co-authored-by: Dongdong Tian <3eae6d0b95aef6af8a0d785fe9f127268703c43c@gmail.com>
Co-authored-by: Wei Ji <9490087a8b98c7de12813b2e194ef86ac3043a88@users.noreply.github.com>
GenericMappingTools/gmt-python,GenericMappingTools/gmt-python